1. Field of the Invention
The present invention relates to a fixing apparatus for melting and thus fixing a toner image on a transfer material in an image forming apparatus. The image forming apparatus involving the use of an electrophotographic system normally includes a fixing apparatus that melts and fixes together a transfer material and toners electrostatically borne on the transfer material, which consist of a resin, a magnetic substance, a coloring material, etc., by applying the heat and a pressure in the way of their being nipped and carried by a press-contact portion (nip portion) between a heating means (such as a roller, an endless belt member, and so on) and a pressure means (such as a roller, an endless belt member and so forth) that rotate in a mutually press-contact manner. The present invention further relates to a fixing apparatus including an external heating means for heating a heating means by abutting on (contacting) this heating means.
2. Description of the Related Art
In the image forming apparatus such as a copying machine, a laser beam printer or a facsimile, etc., what is known as a conventional fixing apparatus for thermally fixing an unfixed toner image transferred onto a recording material, is a fixing apparatus constructed of a fixing roller having a built-in exothermic member like a halogen lamp, etc. and a pressure roller similarly having a built-in exothermic member and brought into a press-contact with this fixing roller, wherein the recording material bearing an unfixed toner image is inserted in between the fixing roller and the pressure roller, the toners softened by a surface temperature of the fixing roller permeate fibers of the recording material, and the unfixed toner image is thus fixed onto the recording material.
The halogen lamps as the exothermic members of the fixing roller and of the pressure roller are controlled ON and OFF based on signals from temperature sensors attached to respective surfaces of the fixing roller and the pressure roller, whereby the surfaces are controlled at predetermined temperatures.
In the thus constructed heating roller type fixing apparatus, a nip portion for conveying the recording material while nipping the recording material, is formed owing to elastic deformation of an elastic layer on the fixing roller. Then, in this type of fixing apparatus, a transfer sheet bearing the unfixed toner image is passed through the nip portion, thereby melting and fixing the unfixed toner image onto the recording material by virtue of thermal energy and pressure.
In the case of the thus constructed fixing apparatus, the fixing roller and the pressure roller have their own elastic layers. Each of these elastic layers is composed of a comparatively-low-heat-conductivity elastic member such as a silicone rubber, etc., having a certain degree of thickness. Therefore, in the aforementioned fixing apparatus, if the surface temperatures of the fixing roller and the pressure roller become lower than the predetermined temperatures during the traveling of the recording material, the respective temperature sensors detect the decreases in temperatures, and the electricity conducts to the respective halogen lamps. Even if so, however, there might be a possibility where deterioration of the fixing occurs because of requiring a considerable period of time until the heat of the halogen lamps is transmitted to the surfaces via the elastic layers. Especially when increasing a process speed, the fixing deterioration occurs easily, and there arises such a problem as to restrict consecutive printing in order to prevent the occurrence of the fixing deterioration.
Such being the case, an already-proposed measure for solving this problem is a technology of bringing, as disclosed in Japanese Patent Application Laid-Open Nos. 10-149044 and 11-24489, an external heating roller kept at a high temperature into a contact with the surface of the fixing roller and thus relieving a decrease in temperature of the fixing roller surface.
In the case of this type of fixing apparatus, however, even when the surface temperatures of the external heating roller and the fixing roller are controlled within proper temperatures, if the external heating roller continues to be in contact therewith during a halt of the rotation of the fixing roller, the external heating roller continuously heats up one point on the fixing roller surface, resulting in a problem that this portion is damaged and deteriorated. Thus, the surface deterioration of the fixing roller leads to an occurrence of problems such as an image defect like gloss deterioration, etc., adhesion (offset) of the toners due to a decline of a releasing property of the surface of the fixing roller, exfoliating and deteriorating the recording material, and so forth.
A construction for coping with these problems is that, as disclosed in Japanese Patent Application Laid-Open No. 11-24465, the conduction to the heater of the external heating roller is kept OFF during the halt of the rotation of the fixing roller.
If the conduction to the heater of the external heating roller is set OFF, however, when an image forming signal is inputted, this requires restarting the conduction to the heater of the external heating roller and heating the external heating roller up to a predetermined temperature. Hence, a process starting from the input of the image forming signal and ending up with the image formation, is time-consuming.
It is therefore preferable to control the temperature of the external heating roller in a way that separates the external heating roller from the fixing roller after finishing the image formation while making the conduction to the heater of the external heating roller be in a standby state, etc., in order to prevent a damage to the fixing roller. This is attained on one hand, and on the other hand there is a necessity of reducing the deterioration of and the damage to the fixing roller surface which are caused by controlling the temperature for a long period of time in a state where the external heating roller abuts on the fixing member during the halt of the rotation of the fixing roller due to an abnormal stop of the rotation driving means, a defective operation of the contacting-and-separating means for making the external heating roller contact and separate, hang-up of software for controlling the rotation driving means and the contacting-and-separating means, and so forth.
Under such circumstances, it is an object of the present invention, which was devised to obviate the problems inherent in the prior arts given above, to reduce, in a way that heats an image heating member in a comparatively short time by controlling a temperature of an external heating member on standby, deterioration of and a damage to a fixing member due to abutment of an external heating member when there occurs an abnormality, such as an abnormal stop of a rotation driving means, a defective operation of contacting-and-separating means for making an external heating roller contact and separate, hang-up of software for controlling the rotation driving means and the contacting-and-separating means, and so on.